Select the default theme in the Thunderbird settings to ensure a consistent appearance.

If you change a Shades-of-gray theme color, it is not necessary to copy the file again. You can simply comment the current theme color in 'userChrome.css' (Thunderbird and Firefox) and uncomment the new theme color value.

• Thunderbird Calendar

Thunderbird‘s Calendar (formaly Lightning) uses its own bright colors in default setting. The option for using theme colors is hidden as 'Accessibility'.
Open the Thunderbird settings and activate the checkbox 'Optimize colors for accessibility' in calendar area.

Note: In Thunderbird 68 Calendar (Lightning) there are several bugs that can cause display errors. In'Shades-of-gray-Readme.html' a workaround is explained.

• Firefox

The directory '../app-gadgets/Firefox-Shades-of-gray' contains a collection of Shades-of gray theme modifications for Firefox:

userChrome.css:
This file contains modifications for the Firefox user interface.

userContent.css:
Herein are adjustments of the Firefox Home-Screen. In addition, the following customizations are imported via 'userContent.css':

Panorama Tab Groups:
This is a modification for the dark theme of the recommended add-on Panorama Tab Groups (AMO).
Note: in Panorama Tab Groups settings you can toggle between light and dark theme. The Shades-of-gray customizations only take effect with the dark theme.

Bookmarked Speed Dial:
Bookmarked Speed Dial (AMO) is an underrated add-on that offers similar functionality to Vivaldi Speed Dial.
The folder 'bookmarked-speed-dial-images' contains images wich you can choose as 'Default dial image' and 'Default folder image' in the add-on options.

To use the customizations for Firefox you can copy the contents of directory '../app-gadgets/Firefox-Shades-of-gray/' to this location:

~/.mozilla/firefox/name-of-your-profile/chrome/

Note: you should choose the dark style in Firefox to get a consistent dark appearance.

Note: As of Firefox 68/69, you must set the 'toolkit.legacyUserProfileCustomizations.stylesheets' preference to 'true in 'about:config in order to load 'userChrome.css or 'userContent.css files.

• Inkscape

Original colored icons in older versions of Inkscape are hard to identify in dark themes. The file 'icons.svg' in folder 'Inkscape-icons' contains bright icons. To use them in Inkscape, the file must be stored in this directory:

~/.config/inkscape/icons/icons.svg

• Syntax Highlighting

Shades-of-gray includes the syntax color scheme Shades-of-pastel for several editors. By using pastel colors on dark background like pastel chalks on a blackboard the Shades-of-pastel scheme provides balanced and easy on eyes contrasts.

Currently Vim, GVim, Sublime-Text and editors with syntax highlighting based on 'GtkSourceView 3' or 'GtkSourceView 4' such as Gedit, Pluma. Xed and Mousepad are supported. Installation notes can be found in 'Shades-of-gray-Readme.html'.

This theme is great. In Gnome 3.36.1 I only have two issues: 1) The Ubuntu store becomes transparent for some reason with this theme turned on; 2) The text in the notification pane is black on dark gray and is hard to read.

My favorite theme for almost 2 years.
Can you make these elements smaller in height?
https://i111.fastpic.ru/big/2020/0402/d6/4883b0c0f99c3b71f5b4ba80041551d6.png
https://i111.fastpic.ru/big/2020/0402/58/bc08adf01d64346eedc8faf06c5def58.png

For me it is probably the best theme. My only issue is that notifications text color is not white (or light grey) but dark grey (almost like background) and it is hard to read. I think it is bug. I am using Pop_Os 18.04 gnome version 3.28.2. I will report this to git hub but if somebody knows how to change that color in .css file, plase let me know. Thank you for this minimalistic feel. Great work.

one of my favorite themes for xfce: it is very clean, not to dark, and it sets the right window decorations for gnome apps, making the whole desktop look very consistent. My only issue is that the accent colors (harvest, patina and the rest ) are not getting applied to the workspace switcher on the xfce panel. Instead, selected workspace always stays dark gray, what makes it hard to see on which workspace I currently am.

Please report bugs, issues and feature requests on GitHub (link above).
I've just committed relevant changes to the workspace-switcher in Xfce 4.12 and Xfce 4.14.
To enjoy the latest commits, you can download and install the Shades-of-gray pre-release Git version from GitHub.

Anyway I don't use it yet on my Ubuntu 19.04 because of incompatibility with my very favorite file manager (Double Commander aka. doublecmd-gtk, installed through apt, not by snap or appimage).
I don't understand what makes a dark theme work with this application or not but I see that Starlabs and Adwaita-dark are theming it perfectly. I don't know the difference with a theme like yours nor which files should be included or modified, sorry to be of no help.
Do you think that this issue with Double Commander could be fixed? I would be glad to use this theme as I really like it.

You're right, I've tried to update double-cmd from the developer's repository to see if it resolves the issue. But it changed nothing, same problem between 0.9.2 and 0.9.6. Anyway the Marwaita GTK3 theme dev and me are actually discussing it on his Pling theme's page and he helped me to point out in terminal output the lack of lines about theme's loading by Double Commander. So the issue seems to come from something else than the theme itself. The investigation is on the way :)

Great work, thanks a lot for sharing it with us.
Anyway I don't use it yet on my Ubuntu 19.04 because of incompatibility with my very favorite file manager (Double Commander aka. doublecmd-gtk, installed through apt, not by snap or appimage).
I don't understand what makes a dark theme work with this application or not but I see that Starlabs and Adwaita-dark are theming it perfectly. I don't know the difference with a theme like yours nor which files should be included or modified, sorry to be of no help.
Do you think that this issue with Double Commander could be fixed? I would be glad to use this theme as I really like it.
Here's a concrete example:
Shades of Gray Arch > https://i.imgur.com/7kHGd7V.png
StarLabs Dark > https://i.imgur.com/FGDgJHI.png
I've posted a message on application's forum to get more informations about the files to customize but nobody answered for now > https://doublecmd.sourceforge.io/forum/viewtopic.php?t=5769